The experimental and finite element simulation methods were used to investigate the mode conversion, the reflection and the transmission phenomenon at the elbow in small diameter tubes. Excitation signals of different frequency were used to inspect tubes involve elbows of different bending radius. The results show that the mode conversion phenomenon occurs at the elbow and the L(0,1) guided wave converts to F(1,1) guided wave. The orientation of F(1,1) is identical to the extradosintrados direction of the elbow. With the increase of the inspection frequency and bending radius, the reflected F(1,1) mode decreases monotonically while the transmitted F(1,1) mode changes non-monotonically, the reflected L(0,1) mode decreases while the transmitted L(0,1) mode increases. When the inspection frequency and bending radius increase to some extent, the change of inspection frequency or bending radius has little influence on the reflected F(1,1) mode, reflected L(0,1) mode and transmitted L(0,1) mode. The experimental results agree with the simulation results well which validates the truth of the simulation results. The conclusion will provide theoretical guide for the inspection of small diameter tubes involve elbows.